With the progress of digital coherent optical transmission techniques in recent years, high-speed and long-distance optical data transmission having a capacity of 100 Gbit/s per wavelength channel has been commercialized. However, research and development on optical data transmission systems having a larger capacity have been actively carried out in order to cope with the increasing demand for network traffic transfer, and an increase of a transmission capacity by multi-valuing and an increase in a baud rate have been proposed.
In optical data transmission, data communication is performed through an optical fiber. It is necessary to reduce the power of a signal capable of being input to an optical fiber to approximately equal to or less than 10 dBm due to the necessity of avoiding signal distortion due to nonlinear optical effects occurring in an optical fiber. When a transmission capacity is increased by multi-valuing and an increase in a baud rate, energy distributed per bit decreases. For this reason, bit errors easily occur, and a distance over which transmission is possible tends to decrease. In recent years, a communication system which is operable by supporting and switching among a plurality of modulation schemes has also been proposed.
In a communication system operated by switching among a plurality of modulation schemes, it is necessary to set a modulation scheme and the like between terminals. It is desirable that such a modulation scheme is automatically set, from the viewpoint of improving operability.
In addition, as another problem, in a large capacity optical transmission channel exceeding 100 Gbit/s, signal processing such as pre-equalization of transmission path frequency characteristics may be performed on a transmission side. In order to perform appropriate signal processing on the transmission side, it is necessary to feedback parameters, such as transmission channel information obtained on a reception side and the quality of a received signal, back to the transmission side.
In order to realize the above-described automatic setting of a modulation scheme and feedback of parameters for transmission-side signal processing, it is necessary to provide a control channel between terminals. In order to realize automatic setting of a modulation scheme, communication needs to be established prior to connecting of a main signal in the control channel. Further, in order to realize the feedback of parameters for transmission-side signal processing, the control channel needs to provide high reliability even when stable communication cannot be established due to frequent occurrence of errors in a main signal.
An example in the related art of a control channel satisfying such requirements is disclosed in Patent Literature 1. In the technique disclosed in Patent Literature 1, a waveform in which energy locally exists in a frequency region is used for the transmission of a control signal and is time-multiplexed to a main signal.